1. Field of the Invention
The present invention relates generally to the field of modem circuits, and more particularly, to an electronic inductor circuit suitable for use in the interface circuit of a high-speed modem.
2. Description of Related Art
A prior art electronic inductor circuit 10, also known as a gyrator, is illustrated in FIG. 1. An electronic inductor typically consists of a transistor 12 with an emitter resistor Re 14, further biased by a resistor divider 16, 18. The resistor divider 16, 18 is connected across the rectified voltage Tip and Ring lines, and a large capacitor C2 is placed between the base of the transistor 12 and ground to block any AC signals on the telephone line from modulating the collector current of the transistor 12. The electronic inductor is part of the telephone line interface circuit that connects a modem to a telephone line. In this configuration, the electronic inductor determines the DC operating point of the modem or the telephone line without affecting the AC response of the signal transmitted or received.
To transmit a signal into the telephone line, it is customary to superimpose the signal across the Tip and Ring lines by means of a capacitor, transformer, or both. Since the line impedance is 600 ohms, driving the line generally requires a low-impedance signal driver capable of a voltage swing as large as 5 volts peak-to-peak. This swing is difficult to achieve with standard power supplies of 5 volts, and impossible to achieve when using 3 volt power supplies. Other drawbacks to this circuit include the required switch 20, and the large capacitance required for C1.
The voltage swing problem is generally solved by using two differential transmit drivers of one-half the desired swing, one single driver with virtually rail-to-rail output swing, or one single driver with power supply greater than 5 volts. Another approach is to drive the base of the transistor 32 in the electronic inductor 30 with the transmit signal 14, after decoupling the base from the voltage divider 36, 38 with a suitable resistor 10, as shown in FIG. 2. Similar configurations are currently used for telephone answering machines. In the later configuration, however, the non-linear characteristics of the transistor 32 are reflected into the signal transmitted on the line, and the signal lacks the linearity required for high-speed modem applications.
The present invention is an electronic inductor circuit in which an operational amplifier drives the base of the electronic inductor transistor, and receives negative feedback from the emitter of the transistor. The transistor and operational amplifier combine to form a voltage-controlled current source (VCCS) with respect to loop current. A voltage divider connected across the rectified Tip and Ring voltage (or another node of the circuit at an equivalent voltage) provides a DC reference to the positive input of the operational amplifier, so that the line current automatically increases with an increase in line voltage. A capacitor couples the transmit signal driver to the positive input of the operational amplifier. The voltage feedback to the transistor is generally different for the AC and DC signals, which effectively translates to different AC and DC currents through the loop. A second transistor may also be used to provide increased isolation for the AC modem when the device is on-hook.
In this configuration, the operational amplifier linearizes the voltage signal at the emitter of the transistor (through the negative feedback input), and causes the line current to swing linearly. Specifically, if the collector voltage is sufficiently above Ve, the harmonic content or distortion of the transmitted signal is at least 80 dB below the fundamental signal level.
The present invention may also be configured using a Darlington transistor pair in order to minimize the current required by the operational amplifier driver into the base of the first transistor.